Effect of Zn2+ ions on the conformation of single-stranded polynucleotides polyU and polyC in a wide temperature range at pH 7 was studied by differential UV spectroscopy and by thermal denaturation. The atoms coordinating Zn2+ ions were determined (O4 and N3 in polyU and N3 in polyC). A three-dimensional phase diagram and its two-dimensional components were constructed for a polyC-Zn2+ system. The phase diagram revealed a region in which ordered single-stranded structures, stabilized by Zn2+-mediated cross-links involving N3 atom of cytosine, are formed. The phase diagram also demonstrated that the behavior of the polyC-Zn2+ system is similar to the effect of retrograde condensation observed in some binary solutions of simple liquids. A dependence of Zn2+-polyC binding constant on the metal ion concentration was obtained. The reason why zinc-induced transition of the sequences with adenine-uracil (AU) base pairs from A-form geometry to a metallized m-form requires higher pH compared to the sequences comprised of guanine-cytosine (GC) base pairs is explained. This information can be useful for the development of possible technological applications based on m-DNA.